Microstructure of porous space in cement sheath used for sealing oil wells

The effect of cement slurry modification on the microstructure of the annular space in a wellbore is presented in this paper. An admixture of macromolecular copolymer GS was used, which is usually applied in cement slurry technology to counteract gas migration by the binding cement slurry. The copolymer addition also results in sealing up the cement matrix at the stage of the binding and formation of hardened cement slurry. The modification of a slurry composition with this admixture results in the fact that the cement matrix has low porosity and is strongly compact. Consequently, a significantly higher number of micropores with small diameters are formed than those with larger micropores in the analyzed hardened cement slurry sample. An admixture of a macromolecular copolymer (7 wt.% of the cement) was added and 10% of the latex was removed from the recipe to show the influence of the modified recipe on the microstructure of the cement sheath. Thanks to this modification, the microcement participation could be reduced by 50% with the simultaneously improved quality of the most important technological parameters of fresh and hardened cement slurry. The basic and modified recipes were analyzed. The following parameters of the slurry were determined: rheology, filtration, water settlement, and time of densification. The influence of the modified slurry on the parameters describing the microstructure of the hardened slurry was analyzed for the hardened slurry (i.e., porosimetric porosity and microtomographic porosity). The microstructure of a sample fracture was also investigated. The analyses were conducted for recipes of slurries to be used for sealing wells at a temperature of 40°C and pressure of 10 MPa.